My philosophy is this: don't ever get a frog you don't have cash in hand to pay for. I've considered a loan for frogs before, but I've never come across a circumstance when I felt the pros outweighed the cons....
Here's some of the criteria I use when I loan out animals:
1. Only loan out animals to people you know and trust. If you wouldn't feel right letting them watch your kids, they don't get the animals!
2. Talk about worst-case scenarios ahead of time, and agree to a resolution if the worst happens - before the animals leave your hands.
3. Agree on what will happen to any potential offspring.
4. Make sure the person actually has the experience necessary to have success with the animals in question.
5. Talk about scheduling some kind of regular contact with the person. I've heard of a lot of cases where someone loans animals out, doesn't hear anything for months, then can't get a hold of the person when they need to. Exchange email addresses, physical addresses, and phone numbers.
6. Agree to a time limit on the loan.
7. Make sure the person understands that it is a loan, and you are not giving them the animal. It is still your property!
Zoos have a standardized form that specifies that the animals are returned at the request of either party, a clause stipulating whether or not renumeration is required if something happens to the animal(s), how the offspring (if any are split)(typically odd number animals go to the lender), whether or not the owner has to be consulted before medical treatments are performed on the animal, as well as a schedule of how often the parties are required to communicate about the animals.
I have it spelled out like Zack, the only difference I want the individuals to treat all incoming amphibians for Chytrid and keep separated from general frog collection for a minimum 30 day quarantine. I have only done this with close friends.
I would avoid a loan wherever possible. At the very least follow the above suggestions and in addition make sure it is understood and in writing who gets final say over the the condition of the frog when it is returned. You dont want to get into a situation where you feel the frog is returned to you in poor shape and the borrower does not. I would also ask the borrower to leave me the cash value of the frog to be returned when I get my animal back and am satisfied with the condition its in (if they dont have the money to let you hold, how would they pay you if something were to happen?)
This may sound over the top, but things happen all the time that are beyond our control (power outages, basements flooding, extreme weather conditions, etc.) and you dont want to end up losing a frog for trying to be a nice guy and lend it to someone, so make sure what happens in any event is clearly understood and documented.
Having said that, why not just work out a permenant trade or something? I wouldnt put a frog through the stress of having to catch it and then moving it back and forth unless it was absolutely necessary.
Honestly, I see way too much downside to share a frog. Outside of the ones listed above my chief concern would be a parasite hitching a ride back on my frog. Then when you get your frog returned and you pop it into it's viv then your viv probably has that parasite and it's looking to permeate whatever other potential host that goes in.
Just too risky and too much out of your control to take the chance in my opinion.